Electronic signal amplifier circuit performance is measured by a variety of parameters such as the amount of noise and the degree of linearity of the amplifier circuit. It is desirable to have a relatively low amount of noise while maintaining a relatively high amount of linearity. An amplifier circuit having relatively low amount of noise and a relatively high degree of linearity is a circuit employing an operational amplifier (op-amp) such as part no. CM409 manufactured by Comlinear Corp of Ft. Collinsville, Colo. Of course, there are many other types of amplifier circuits, using other components, which exhibit low distortion and high linearity. However, an amplifier circuit with a high impedance voltage source coupled to the CM409 op-amp would have a relatively high degree of noise due to the amount of current noise of the op-amp interacting with the source impedance.
The noise performance of all amplifiers can be described in terms of a series voltage noise and a shunt current noise at the input of the amplifier. If the source impedance becomes relatively high, then the current noise component (when multiplied by the source impedance) may result in an equivalent noise voltage that will significantly increase the noise levels at the amplifier input.
When a high source impedance must be used, the use of an FET preceding the amplifier will make a major reduction in the current noise term, thus driving the amplifier from a high impedance source without a major noise increase. However, FET devices have a disadvantage in that they can add significant amounts of distortion.
There is a need for a highly linear amplifier circuit having a relatively high input impedance and a relatively low noise.